One of the most widely used detectors in gas chromatography is the thermal conductivity detector (TCD) or the katharameter In these detectors, changes in thermal conductivity of the carrier gas is measured by the thermistor. A thermistor is a small lead prepared by fusing oxides of metals.
The carrier gas used is either helium or hydrogen, Sometimes, nitrogen has also been used as the carrier gas because helium is costly and hydrogen is dangerous. But the sensitivity is greatly reduced. In the detector two pairs of matched thermistors from the wheatstone bridge circuit are arranged.
Carrier gas is passed over one pair (reference) while a mixture of carrier gas and column effluent is passed over the other pair (sample) of thermistors.
When pure carrier gas passes over both the pairs of thermistors, the bridge is balanced.
But when the mixture of carrier gas plus effluent gas passes over one pair and pure carrier gas over the other, then the bridge is unbalanced because of unequal cooling of the two pairs of thermistors.
The extent in imbalance is a measure of the concentration of the column effluent in the carrier gas.
The imbalance is fed to a recorder to give the chromatogram. As can be easily seen this is a differential technique based on the difference in thermal conductivities of the reference and sample. Thermal conductivity detectors are comparatively simple, sturdy and reliable and its sensitivity is sufficient for most purposes.